1. Field of the Invention
The present invention relates to a diaphragm apparatus for controlling a light amount in an optical apparatus such as a camera or the like and, more particularly, to a full closed type diaphragm apparatus which can completely close an aperture.
2. Related Background Art
The shape of diaphragm aperture in a photographing lens of a camera exerts an influence on the integrated evaluation of delineating characteristics of the lens. That is, a feature of an out-of-focus state at the background of a main object to be photographed slightly differs for every aperture shape of the diaphragm. In a ghost phenomenon in which the image of the diaphragm is projected onto a film, the aperture shape itself is photographed. Thus, a shape near a true circle is generally preferable as the shape of an aperture by the diaphragm. Therefore, it is desirable that the number of blades is large. It is also preferable that the aperture shape is unchangeable for a change in diaphragm aperture.
In the region near the lens mirror barrel in which the diaphragm is assembled, generally, there is no abundance of space for the structure. Therefore, a large number of blades makes design and manufacture difficult. As the number of blades increases, when the diaphragm blades are collected toward the center of the opening portion to reduce the aperture, the edge surfaces of the diaphragm blades collide and cause friction. There easily occurs a phenomenon such that the further movement of the diaphragm blades toward the center of the opening portion cannot be performed and it becomes impossible to reduce the diaphragm to a further smaller value. Such a phenomenon can be relatively easily solved in the case of a diaphragm apparatus which does not execute the full closing operation. However, in a full closed type iris diaphragm apparatus which needs the full closing operation and is assembled in, for instance, a lens for a television camera, such a phenomenon easily occurs when the diaphragm is fully closed. Further, as the number of diaphragm blades increases, the difficulties in designing and manufacturing are increased and it is difficult to avoid the phenomenon.
Hitherto, an apparatus comprising six diaphragm blades has been known as a full closed type diaphragm apparatus whose aperture shape is unchanged even if the diaphragm value changes. An apparatus comprising seven blades is more preferable than the diaphragm apparatus comprising six blades because the aperture shape is closer to the true circle. However, in the case of the conventional full closed type diaphragm apparatus using seven diaphragm blades, the apparatus has one to three blades only for use in the closing operation and in the case of further reducing the diaphragm from a certain diaphragm value, for instance, from the minimum diaphragm value, such closing blades are rotated to close in accordance with the diaphragm reducing operation. When the closing blades start operating, the regular heptagonal aperture shape is destroyed and the resultant aperture shape differs from the inherent shape. To construct the diaphragm so that the aperture shape does not change until the full closed state, it is necessary to eliminate the phenomenon whereby the diaphragm blades collide and cause friction due to the diaphragm reducing operation and whereby the diaphragm cannot be further reduced before it is completely closed. However, the use of seven blades makes it very difficult to design and manufacture such a construction.